You have £100, right? And you spent £97. With £3 change. £97 + £3 = £100 You give £2 back, leaving £98 pounds that you still owe them. Where this money is does not matter: it is still inclusive in the money borrowed, and so is the £2 you gave back, as it was part of the original amount borrowed. Therefore;£2 + £97 + £1 = £100